#include <iostream>
#include<cstring>
#include<string>
#include<vector>
#include<algorithm>
#include<time.h>
#include<stdio.h>
#include<stdlib.h>

String randstr(){
    srand(int(time(0)));
    String str_list[] = {
        "a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","0","1","2","3","4","5","6","7","8","9"
    };
    int length = sizeof(str_list) / sizeof (str_list[0]);
    return str_list[rand() % length];
}